Fixed the issue. Added the below in Plesk Onyx under Apache & nginx Settings > Additional nginx directives
fastcgi_buffers 8 16k;
proxy_buffers 8 16k;
If the above solution doesn't work for you, edit Workice file /path-to-workice/app/Http/Kernel.php around line 30, comment this line as shown;
Hope this helps if someone else who has the same issue.